Feature 3
hull mode at 3.6Hz. However, the extensive Equivalent excitation force to about 3.6Hz, occurred at all. To clarify this
produce 1 mm/s transverse
tests could not pinpoint a specific location
Force x/L-Pos. vibration in the owner's cabin
point a CFD analysis was put forward as
Vertical
of the unpleasant vibration source.
application Point Transv. Direction Direction
the second pre-investigation.
[kN] [kN]
The speed-up manoeuvre shown in Fig
aft bow thruster 0.87 10 22
fwd bow thruster 0.90 10 16
1 clearly shows the characteristics of the
fwd exhaust gas outlet PS 0.41 9 33
CFd analysis
vibration problem.
fwd exhaust gas outlet SB 0.41 9 33
Systematic CFD simulations were
fwd sea chest PS 0.40 9 50
Starting at about 2.5Hz, the excitation
fwd sea chest SB 0.40 9 48
performed, analysing the flow around the
in question led to high amplitudes in
aft exhaust gas outlet PS 0.31 12 9
motor yacht. The first objective of the CFD
aft exhaust gas outlet SB 0.31 12 6
the owner’s cabin when crossing the
aft sea chest PS 0.22 15 50
analysis thus was to determine areas where
torsional hull mode shape at about 3.6Hz,
aft sea chest SB 0.22 15 13
vortex shedding occurred at a ship speed
outer shaft 0.16 8 4
corresponding to 16knots-18knots ship
inner shaft 0.15 6 6
of about 16knots and then - following the
speed. At higher ship speeds the vibration
inner I-bracket fwd 0.08 4 6
sensibility study - eventually to confirm the
outer I-bracket 0.08 5 2
response was still present but lower in
skeg 0.04 2 39
source of the 3.6Hz excitation somewhere
amplitude as the resonance area had been
inner I bracket aft 0.03 4 6
in the aft part of the vessel.
outer V-bracket 0.03 4 2
passed.
inner V-bracket -0.02 4 5
The analysis focused on the detailed
When comparing this response to the
bow thruster tunnels, see Fig. 4. Further the flow
f
in
lo
the
w d
neighbourhood
escription ne
of
ar
t

h
t
e
h
p
e
r

o
h
p
u
e
l
ll
l
e

r
a
sh
nd
a

ft
t
s
h
a
e
nd the
Fig 3:
Fig.
vibration response at the lower right corner
propeller
3 Excitation
excitation forces required to match
brac
Forces
kets
required
was
to match
computed
the vibration level
to
measu
investigate
red
the
ske
flow
g. T
interaction
he comp
between
utationa
these
l volu
components.
me mesh

the vibration level measured. Obviously the 1-node torsional vibration mode is most effectively excited by torsional moments acting on
of the figure the difference in character the ship’s aft body (low force values). This moment can be produced by either transvertseh or atvertical was generated modelled the hull, the
forces acting on the hull girder. Vertical forces are considered more probable since hydrodynamic
became obvious: the excitation in question pressure variations act on a comparatively large and flat hull area. skeg, and the exhaust gas outlets, as well
was not of this mechanical type, which The next question was whether vortex shedding having a frequency of about 3.6 Hz occurs at all. To as the bow thruster tunnels, as shown
clarify this point a CFD-analysis as a second pre-investigation was put forward.
clearly followed the stepwise revolution in Fig 4. Furthermore, the flow in the
CFD-Analysis
increase of the power train. neighbourhood of the propeller shafts and
Therefore, systematic CFD simulations were performed analysing the flow around the motor yacht. The
So, knowledge of the vibration was first objective of the CFD analysis thus was to determine areas where vortex shedding toccurs he atp ra oship peller brackets was computed to
speed of about 16 kn and then -following the sensibility study- eventually confirm the source of a 3.6 Hz
almost complete: the mode shape (form) excitation somewhere at the aft part of the vessel. investigate the flow interaction between
and the frequency were known, as well as The analysis focused on the detailed flow description near the hull and the skeg. The computthesatioe nal components.
volume mesh that was generated modelled the hull, the skeg, the exhaust gas outlets as well as the
the fact that resonance at a certain ship A Reynolds-averaged Navier-Stokes
speed occurred and (very importantly) that equation (RANSE) solver was used. The
the vibration problem was reproducible.
Fig 4: Fig. 4s urface grid for CF Surface grid ford C analysis. FD analysis
conservation equations for mass and
This knowledge would have formed a momentum in their integral form served
good basis to solve the problem; if only the as the starting point. The solution domain
number of possible excitation mechanisms w
Numerical
as then co
Simulation
mputed at the owner’s cabin at was subdivided into a finite number of
were more limited. a
A
n
Reynolds-averaged
interval of around 3
Nav
.6H
ier-Stokes
z.
equation (RANSE)
control v
solver
olume
was
s th
used.
at ma
The
y be
c
o
onservation
f arbitrary
equations
It was for this reason that the yard
for
Th
mass
e tran
and
sver
momentum
se response
i

n
i

s
their
illust
integral
rated in
form
sh
se
ap
rve
e.
as
The
the
in
starti
tegr
ng
als
point.
were
The
num
solution
erically
domain is
preferred to order pre-investigations rather F
subdiv
ig 2 in
ided
an
into
exem
a
p
finite
lary w
nu
a
mber
y for s
of
om
control
e area
volumes
s appr
that
oxim
may
ated
be
us
of
in

g
arbitrary
the mid
shape.
point ru
The
le. Th
integrals
e
are
than rely on the trial and error principle. o
numerically
f possible
approximated
vortex shedd
us
in
ing
g g
the
ene
midpoint
ration.
rule.
ma
The
ss fl
mass
ux th
flux
rou
th
gh
rough
the c
the
ell
cell
face
face
was
is
ta
taken
ken
from the
In a first step, a vibration sensibility Thprevious e mostiteration, significafollowing nt respona sseismple aros Pice fraord m iteration fromapproach. the preThe viouunks itnown eratiovanriables, follo at withe ng center of
study was performed on the basis of an FE- v
the
ertic
cell
al e
face
xcitat
are
ion
determined
at the outer
by
sha
com
ft li
b
n
ining
e -
a
a
cent
sim
ral
ple
differencing
Picard iter
scheme
ation ap
(CDS)
proach
with
. Th
an
e
upwind
model aiming at those areas where vortex th
differencing
e transvers
s
e
cheme
excita
(UDS).
tion at
A
th
two-equation
e aft edge o
turbulence
f unkno
model
wn var
was
iab
used.
les at the centre of the
shedding theoretically may be exciting the the skeg. cell face were determined by combining
The ship was treated as a double body, consisting of the ship hull below the calm-water surface and its
hull girder.
mirror
Vibr
image
ation
above
levels
the
due
calm-water
to the uni
su
t
r
f
f
o
ac
r
e
ces
. The
a
fr

e
c
e
e

n
su
tr
r
al
fa

c
d
e
i

ff
w
e
a
r
s
e
n
n
o
c
t
i

n
considered.
g scheme (
Veloc
CDS
i
)
ties
wit
that
h
th
initialize
en yield
the
ed
flow
the e
field
quiv
arose
alent
from
force
the
s re
inverse
quired
of ship
an u
speed
pwind
imposed
differen
at
c
the
ing
inlet
sche
boundary.
me (UDS
The
). A
three-
vibration sensibility study to
dimensional
excite the m
flow
eas
field
ured
surrounding
transverse
the
vib
ship
ratio
was
n
com
two
p
-
u
e
t
q
e
u
d
a
a
ti
s
o
a
n


t
t
r
u
an
rb
si
u
e
l
n
e
t
n
p
c
r
e
o

c
m
es
o
s
d
.
e
T
l
h
w
e
a
f
s
lu
u
id
s

e
d
d
o
.
main was
For a start the free vibrations of the yacht le
id
v
e
e
a
l
l
o
iz
f
e
1
d
m
by
m
a
/

s
v

o
in
lu
t
m
he


g
o
r
w
id
n
c
e
om
r’s
p
c
r
a
i
b
si
in
g
.

Th
abo
is
u

t
i

s
6 millio
Th
n h
e
e

xahedral
ship was
co
tr
ntr
ea
ol
te
volu
d as
mes
a do
(cells).
uble b
To
od
avoid
y,
flow
were calculated using a 3D finite element ddisturbances epicted in Fiat g 3grid . boundaries, they were located conat sisufficiently sting of thlare sge hipdistances hull beloahead w theof cathe lm-bow, aft
model. In the frequency range of interest,
of
O
the
bv
stern,
iously
and
th
beneath
e one-
the
no
keel.
de torsional water surface and its mirror image above
the measured one-node torsional mode vibration mode is most effectively excited the calm-water surface. The free surface was
shape at 3.6Hz was clearly established. b
Results
y torsion
of
al
CFD-Analysis
moments acting on the ship’s not considered. Velocities that initialised
Subsequently, harmonic unit excitation a
The
ft bo
simulations
dy (low fo
revealed
rce value
v
s
o
)
rtex
. Th
shedding
is mome
at
nt
the
t
bow
he fl
thruster
ow field
tunnels
arose fr
and
om
at
th
the
e in
exhaust
verse of
gas
ship
outlets. A
forces were applied at the locations of c
stationary
an be pro
vortex
duced
was
by
observed
either tr
port
ansv
side
erse
at
o
the
r
tr
s
a
p
ili
e
n
e
g
d
e
i
d
m
ge
p

o
o
s
f
e
th
d
e
a
s
t
k
th
eg
e
n
i
e
n
a
l
r
e

t
th
b
e
o
k
u
e
n
e
d
l.
a
H
r
o
y.
w
Th
eve
e
r, no
possible vortex shedding generation, ie v
vortex
ertical
shedding
forces a
was
ctin
observed
g on the
there.
hull
Pronounced
girder. 3D
bow
flo
th
w
ruster
field
vortex
surro
shedding
unding t
was
he s
revealed,
hip was
but the
at sea chests, exhaust gas outlets, bow V
corresponding
ertical forces
fluctuating
are consid
force
ered
amplitudes
to be mor
occurr
e c
ed
om
at
pu
a
te
quite
d as
lower
a tran
frequency.
sient proc
The
ess.
frequency
The fluid
of the
thrusters, stern tubes, shaft brackets, and livortex kely, shedding since hat ydthe roexhaust dynamipipesc pr was essucomputed re domto aibe n about was id1.4 ealHz. ised by a volume grid
the aft edge of the skeg. The excitation variations act on a comparatively large and comprising about 6 million hexahedral
forces were applied separately for each fl
As
at h
these
ull ar
results
ea.
did not allow to pin point the possi
co
ble
ntr
s
o
ource
l vol

u
of
m
excitation
es (cells
the
). T
numerical
o avoid
simulation
flow
was
location, both in the transverse and vertical
deviated
The nex
to
t q
the
ues
flow
tion
surrounding
to address w
the
as w
ship’s
heth
ster
er
n in the vicinity of the propeller shaf
disturbances at grid boundaries
ts.
, they were
ship’s direction. The forced vibration level v
The
orte
computations
x shedding,
showed
having
that
a fr
the
equ
flow
enc
at
y
the
of
propeller
located
tunne
at su
l
ffi
and
cie
surrounding
ntly large di
the
stan
pr
c
op
es
elle
ahe
r
a
shaf
d
ts was
The Naval Architect March 2008
characterized by the formation of vortices. To help visualize this vortex shedding, Figs. 5, 6 and
65
7 show
samples of computed flow velocities in the neighborhood of the propeller tunnels. The vortex shedding
frequency was computed to be 3.8 Hz for the outer tunnels, for the V-brackets and the I-brackets, and
for the outer shafts. So, at least one of these components was to be considered as a possible exciting
source for the 1-node torsional hull vibration at 3.6 Hz.
NA Mar 08 - p64+65+67+69.indd 65 10/03/2008 14:28:36
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